1. Field of the Invention
This invention relates generally to methods and apparatus for measuring the power of a laser, and relates more particularly to a laser power meter having an absorber mass and a circuit that monitors the temperature of the absorber mass and compensates for convection losses during a power test.
2. Description of the Relevant Art
One way to measure power or energy in a laser beam is to expose a mass to the laser beam and measure the amount of heat absorbed by the mass over a period of time. The mass absorbs incident laser radiation and converts the optical energy of the laser beam into heat. This heat ultimately flows to a heat sink that is held at a near constant ambient temperature by either air or water cooling. The temperature difference between the absorber mass and heat sink is converted into an electrical signal by a temperature sensor, such as a thermocouple or thermistor. In order to obtain a measurable temperature difference between the temperature sensor and the heat sink, the device is designed for controlled thermal resistance between the two.
Standardized calibration measurements have been made for lasers using isoperibol calorimeters to measure laser power or energy. An isoperibol calorimeter is one in which an absorbing cavity is surrounded by a constant temperature environment. The laser beam heats the absorbing cavity and temperature sensors determine the resultant thermal rise. Isoperibol calorimeters account for radiation and conduction of the absorbed heat, but are constructed to minimize and ignore heat losses by convection.
U.S. Pat. No. 5,316,380 discloses a laser power meter with an energy absorbing mass, a device for measuring the change in temperature of the absorber, and an electronic timer. The electronic timer defines a period of time over which a change in temperature of the absorber is measured. A disadvantage of this type of power meter is that it ignores convection losses from the absorber. To compensate for this shortcoming, the absorber needs to start at room temperature in order to obtain accurate and consistent readings. Starting a measurement with the absorber at an elevated temperature would introduce a source or error due to the temperature dependence of convection losses.